The present invention relates to an optical pickup which is provided in a compatible system for reproducing a CD, CD-Recordable (CD-R), and Digital-Versatile Disc (DVD).
The development of the DVD, which is a high-density recording medium, has come to be much in progress. The DVD has a much larger capacity than the conventional CD so that one whole movie can be recorded on the disc. Under such a circumstance, a compatible reproducing system for reproducing both the CD and the DVD is much sought after.
Comparing the constructions of the DVD and the CD, the distance from the surface of the DVD to the information recording surface, that is the thickness of a protection layer of the disc, is 0.6 mm, which is about half as much as that of the CD. If an optical pickup adapted to converge a light beam at a focal point appropriate for the DVD is used when reproducing a CD, due to the difference in the thickness of the protection layer, there occurs a wave front aberration such as a spherical aberration in the light beam. Hence the converged light beam appropriate for the DVD cannot be accurately applied to the CD.
In addition, the size of information pits formed on the disc for recording information differs in the CD and the DVD. More particularly, the length of the smallest pit formed on the CD is about 0.87 .mu.m, while the length of the smallest pit formed on the DVD is about 0.4 .mu.m. In order to accurately read the information pits, it is necessary to apply a beam spot appropriate for the pit in each disc.
It is further preferable to be able to reproduce a write-once CD-R with the compatible reproducing system. However, the reflectance and absorption characteristics of the material comprising the recording layer of the CD-R depend largely on the wavelength of the applied light beam. Namely, it is necessary to provide a laser diode which emits a light beam having a wavelength about 785 nm for the CD-R. On the other hand, the DVD requires a light beam having a wavelength of 635 nm. Thus, a sufficient reflectance for reading out signals from the CD-R cannot be obtained by the light beam emitted from the laser diode for the DVD. As a result, the CD-R cannot be reproduced.
In order to solve the problem, it suffices to provide a first optical pickup having an appropriate optical characteristics for the DVD and a second optical having an appropriate optical characteristics for the CD and the CD-R. However, the number of parts in such a reproducing system with two pickups is increased, thereby inevitably enlarging the reproducing system and increasing the manufacturing cost thereof.